This invention relates to the exploitation of subterranean formations using drilling, drill-in, completion, work-over, packer, well treating, testing, spacer, fluid loss control or hole abandonment fluids. More specifically, this invention is directed to a method of enhancing wellbore treatment fluids, particularly fluids used in deep wells, by enhancing the thermal stability of the drilling fluid and reducing drilling fluid loss to surrounding strata. A fluid for use in the present invention comprises a weighting agent, a viscosifier and a solvent. The solvent, which includes a polyol, e.g., a glycerol, glycol or polyglycol, provides a medium that increases fluid viscosity, dissolves a variety of weighting agents and enhances the thermal stability of the fluid. The fluid optionally includes surfactants, buffering agents, filter control agents, and weight-up agents.
There are several different types of drilling fluids used in the exploitation of subterranean formations; each fluid is specifically prepared for a particular drilling operation or wellbore environment. All drilling fluids contain additives to impart desired physical and/or chemical characteristics to the fluid. Typically the fluids contain Theological additives, fluid loss control additives and weighting agents (either dissolved or suspended solids). The Theological additives include lubricants, viscosifiers, and clayey material to lubricate the drill bit, drill string and related equipment. In addition to lubricating drill bits, drill string and related equipment, the viscosifiers and clayey material also serve to suspend solids and help xe2x80x9cfloatxe2x80x9d cutting debris out of the wellbore. Viscosifiers can also be classified as fluid loss control additives. However, fluid loss control additives also include bridging agents and/or sized particles to prevent loss of the fluid to the neighboring formation. When used as a fluid loss control agent, viscosifiers provide a fluid with sufficient viscosity to inhibit seepage of the fluid into the subterranean strata. Weighting agents typically include salts such as barite (barium sulfate), sodium bromide, sodium chloride, potassium chloride, calcium chloride, calcium bromide, zinc bromide and mixtures of these salts. The weighting agents provide the fluid with sufficient density so the hydrostatic pressure of the dense fluid in the wellbore counterbalances pressure exerted by the fluid in the strata. An optimum fluid provides constant lubricity under the high shear conditions generated by the rotating drill bit, is sufficiently viscous to prevent fluid loss into the formation, suspends solids and xe2x80x9cfloats upxe2x80x9d or removes the debris from the wellbore.
It is difficult to maintain a fluid having the desired lubricity and viscosity under the extreme shear, pressure and temperature variances encountered during drilling operations, especially when drilling very deep wells that descend 15,000 to 30,000 feet (4,500 to 10,000 meters) or more below the earth""s surface. Under these conditions many of the viscosifying agents, particularly polysaccharides such as starch, cellulose, galactomannan gums and polyacrylates, are not stable at such high temperatures and tend to un-crosslink and de-polymerize, thus losing their effectiveness. The degraded polysaccharides can cause the drill string to bind in the wellbore and induce formation damage. Thus, there is a need to enhance thermal stability of drill fluids, especially fluids that include polysaccharide based viscosifiers.
Loss of fluids is also a major concern for well operators. Fluids can be expensive to replace. The fluids lost to the formation often deposit solids on the formation wall adjacent to the wellbore. The deposited solids can cause formation damage and reduce the permeability of the formation to the subsequent out-flow of oil, gas, condensate or other fluids targeted for withdrawal and use. Furthermore, these deposited solids also can cause a drill string to bind in the wellbore.
In specific situations, such as during perforation of the well casing, it is considered particularly advantageous to incorporate a fluid loss pill in addition to the normal fluid loss control additives typically included in the wellbore treatment fluids. The fluid loss pill is a specific fluid that is injected into the well and designed to alleviate the fluid loss, particularly from completion fluids, into the formation. There are two basic types of fluid loss pills: solids-free, and solids-containing fluid loss pills. Solids-free pills typically contain water-soluble polymers to increase the viscosity and, hence, reduce the fluid loss. The solids-containing pills contain non-dissolved or suspended solids such as minerals and salts. The solids are deposited on the formation wall and form a substantially impermeable filter cake. Since solids-containing pills deposit solids directly on the formation wall, which can cause formation damage, they are less preferred to the solids-free pills.
Solids-free pills include polymers to increase the viscosity of the drilling fluid to reduce fluid loss. The polymers used as viscosifying agents are not deposited as solids on the formation walls, and therefore, the solid-free fluid loss pills induce less formation damage and can be more readily removed from the wellbore to initiate well production. However, as discussed above, use of polysaccharide viscosifying agents is limited to temperatures below about 150xc2x0 to 200xc2x0 F. (65xc2x0 to 93xc2x0 C.), which are typically found in wellbores less than about 15,000 feet (4,500 meters) below the earth""s surface. These polymers tend to break down and de-polymerize under the extreme temperatures in deep wellbores. Current trends of drilling increasingly deeper wells in search of additional reserves of oil, gas and other resources require new methods of enhancing the thermal stability of the drilling fluids and reducing fluid loss into the surrounding strata.
Thus, there is provided in accordance with the present invention a method of enhancing the thermal stability of a fluid for drilling, drill-in, completion, work-over, packer, well treating, testing, spacer, or fluid loss control that includes polymers such as polysaccharides. The method includes providing said fluid that includes a polyol, a viscosifying agent and a weighting agent. This fluid for drilling, drill-in, completion, work-over, packer, well treating, testing, spacer, or fluid loss control is added to the wellbore and a polyol concentration of greater than about 15 wt % based on the fluid is maintained in the wellbore. The fluid is particularly useful in very deep wells that exert extreme pressure and temperature on wellbore treatment fluids. Use of this fluid for drilling, drill-in, completion, work-over, packer, well treating, testing, spacer, or fluid loss control provides a fluid that does not significantly change viscosity under the extreme conditions found in very deep wells. Furthermore, use of said fluid provides a fluid that inhibits stress cracking and pitting corrosion on the carbon and stainless steel components of the drill strings, well-drilling and related fluid handling equipment.
There is also provided in accordance with the present invention a method of reducing the fluid loss from a wellbore into neighboring subterranean formations. The method includes employing a fluid loss pill comprising a weighting agent and a polyol. Preferably, the polyol is selected from glycerol, a glycol or a polyglycol. The fluid loss pill is maintained in the wellbore adjacent to the formation wall to reduce fluid loss from the wellbore. The fluid loss pill also can include a viscosifier or supplemental fluid loss control starches or polymers. In one embodiment of the present invention, the fluid loss pill is substantially water free. Use of the fluid loss pill prepared in accordance with this invention reduces fluid loss from the wellbore and minimizes formation damage. In addition, the fluid loss pill exhibits a broad range of compatibility with a variety of formations and wellbore fluids such as shales, seawater, fresh formation waters, oils, completion fluids, packer fluids and spacer fluids.
There is also provided a fluid loss pill for reducing fluid loss from the wellbore. The fluid loss pill comprises a polyol, a weighting agent and a viscosifier. The polyol is selected from glycerol, glycols or polyglycols. The weighting agent and viscosifier can be selected from any of the weighting agents and viscosifiers known and/or commonly used in the art.
Accordingly, one object of the present invention is to provide a method of enhancing the thermal stability of a wellbore treatment fluid that includes a polymeric viscosifying agent.
It is another object of the present invention to provide a method of reducing the amount of fluid loss from a wellbore by injecting into the wellbore a fluid that includes a fluid loss pill that includes a polyol.
It is yet another object of the present invention to provide a fluid loss pill to inhibit loss of fluid into formation strata.